The subject matter described herein relates generally to analyzing a substance and, more specifically, to performing an analysis of a substance using a portable spectrometer.
Portable spectrometers, also referred to as analyzers, are used to examine the composition of a sample material in a number of applications. For example, portable spectrometers are used for metal identification, detection and identification of hazardous materials or explosives, detection and identification of environmental pollutants, and identification of lead in paint. Portable spectrometers may also be referred to as handheld spectrometers if the portable spectrometer is configured for handheld operation.
Examples of specific portable analyzers include X-ray fluorescence (XRF) spectrometers and ion mobility spectrometers (IMS). XRF spectrometers detect secondary radiation emitted from a sample of material that has been excited by radiation applied to the sample material by the spectrometer. A wavelength distribution of the emitted radiation is characteristic of the elements present in the sample, while the intensity distribution gives information about the relative abundance of the elements in the sample. By means of a spectrum obtained in this manner, an expert typically is able to determine the components, and quantitative proportions of those components, within the examined test sample.
An IMS analyzes ion mobility to determine the composition of a sample material. Ion mobility analysis measures the movement of ionized sample molecules in a uniform electric field through a given atmosphere. Once a spectrum is obtained corresponding to the measured ion mobilities, a composition of the sample material can be determined.
A full-sized laboratory diffraction analyzer typically includes a collimator having three sets of slits or apertures. The slits or apertures are typically defined within plates. Radiation striking the plate beyond the margin of the aperture is deflected or absorbed. The beam projected through the aperture has a cross-sectional shape similar to that of the aperture and a size or diameter controlled by the position and size of the aperture, and the position of the source. More specifically, the first slit is located near the radiation source and defines the spot size. The spot size is, for example, a diameter of a spot on a sample illuminated by the radiation. The second slit is positioned so it slightly cuts into the radiation beam and is of a size similar to the first slit. The first and second slits collimate the source beam. The third slit is located between the second slit and the sample and is of a slightly larger size than the first and the second slits. The third slit removes the scattered radiation that is emitted from the second slit.
Typically, the size of such a three-slit collimator prohibits use in a portable analyzer, and even more so, in a handheld analyzer. A portable analyzer may use a pipe or a tube that defines an opening to direct generated radiation toward the sample. In the case of an XRF spectrometer, the pipe or tube is typically more for shielding all but the sample from the generated radiation than it is an attempt to collimate the generated radiation. For analyzers that include a laser source, an external collimator may be used to control the spot size, but collimation is unnecessary since the emitted radiation is inherently well collimated.
Furthermore, a typical three-slit collimator is not suitable for use in a portable or handheld analyzer because the three-slit collimator increases a distance between the radiation source and the sample. More power is required to maintain a flux at the sample as the distance between the radiation source and the sample increases. Therefore, it is generally beneficial, especially in portable or handheld analyzers where available power may be limited to power provided by a battery, to minimize the distance between the radiation source and the sample. For similar reasons, the detector is also positioned close to the sample.
The field of view of a detector with this type of geometry is typically large. Such small distances between the radiation source and the sample may cause problematic XRF scattering, Compton and Rayleigh back scatter that increases background noise, Bragg peaks that are not consistent from sample to sample, as well as sample orientations that result in fictitiously high results. Additionally, scattered radiation from the collimator may excite atoms present in the instrument and the resulting fluorescence may be received at the detector, causing an inaccurate reading.